High molecular weight wholly aromatic polyamides or aramids made by the condensation or reaction of aromatic or essentially aromatic monomeric starting material or materials described in U.S. Pat. No. 4,198,494 and sold under the trademarks Nomex by E. I. duPont de Nemours and Co., Conex by Teijin Corp., and Apyeil and Apyeil-A (Apyeil containing finely divided carbon) by Unitika Ltd. are extremely strong and durable and have excellent flame resistant properties. Shaped articles made of these aramid fibers such as yarn and textile fabrics are commercially important and gaining in popularity especially in the protective fabric field and other markets where the combined flame resistance and high tensile properties are essential.
A serious problem limiting the full commercial exploitation of the aramid fibers has been the fact that fabrics made from these highly crystalline fibers of extremely high glass transition temperature are very difficult to print into colored patterns and designs with good overall fastness properties, especially to light and washing, without adversely affecting their handle, tensile, and flame resistant properties.
Recently, it has been proposed in U.S. Pat. No. 4,525,168 to print aramid fabrics with anionic dyes, i.e. acid dyes, premetalized acid dyes, and direct dyes. This is accomplished by introducing into the aramid fiber dye site substances such as aromatic and aliphatic amines capable of forming ionic bonds with anionic dyes. The dye site substances are introduced and fixed inside the fiber by a special process prior to the printing operation. After printing the fabric with anionic dyestuff and drying, the printed fabric is turbo steamed under pressure to penetrate and fix the anionic dyestuff inside the fiber.
This process suffers a number of technical and economic drawbacks. It requires a special pretreatment process involving the use of specialty chemicals to provide the fiber with dye sites. Only anionic dyestuffs, i.e. dyestuffs containing one or more sulfonic acid groups or their sodium salts, can be used in the printing operation. Furthermore, it requires turbo steaming, a non-continuous operation to penetrate and fix the anionic dyes inside the fiber in order to develop the true shade and fastness properties of the prints. Further, experienced operators report that turbo steaming of printed fabrics tends to give rise to track-off problems in production.
In another development it has also been proposed by Cook and co-workers, Effect of Auxiliary Solvents in STX Coloration of Aramids and PBI with Cationic Dyes in "Book of Papers, AATCC National Technical Conference," New Orleans, La., Oct. 5-7, 1983, pp. 314-326, to improve the screen printing of Nomex aramid fabrics. In the procedure described the Nomex aramid fabric is pretreated in certain highly polar solvents such as DMSO under suitable conditions, i.e. pad-squeeze, heated at 150.degree. F. for 10 minutes, washed at 100.degree. F. and dried prior to the printing operation. In this case too, the fabric has to be pretreated in a special process prior to the printing operation as outlined above. Furthermore, such pretreatment if not properly controlled, may cause drastic reductions in the tensile and mechanical properties of the fabric.
Accordingly, it is an object of the present invention to provide an improved process for the printing of aramid fabrics. Another object of the invention is to provide a method whereby fabrics made of aramid fibers can be printed with a variety of conventional organic dyestuffs such as cationic, anionic, disperse, fiber reactive, solvent, vat, azoic, dyes as well as mixtures thereof to obtain printed patterns with superior overall fastness properties. The process allows the use of two or more dyestuffs of different classes in the same print paste formulation, and this is believed to be unique. Still another object of the invention is to provide an improved process for the printing of aramid fabrics in which penetration and fixation of dyestuffs inside the aramid fiber are achieved Finally, another object of the invention is to provide an improved process for the printing of aramid fabrics whereby the curing of the printed goods is carried out continuously under atmospheric pressure. Other objects of the invention will become apparent from a consideration of the description which follows.